1. Field of the Invention:
This invention relates to an ultrasonic Doppler blood flow velocity detection apparatus and to a method for detecting blood flow velocity.
2. Description of the Prior Art:
An ultrasonic Doppler blood flow velocity detection apparatus is used for detecting blood flow velocity which can be used in diagnoses. There are many types of Doppler blood flow detection apparatus utilizing the Doppler effect through reflection of ultrasonic waves. The ultrasonic Doppler blood flow velocity detector can select a portion where blood flow is to be detected with respect to distance and direction.
The most popular Doppler blood flow velocity detection apparatus detects blood flow velocity as follows:
The Doppler blood flow velocity detection apparatus transmits an ultrasonic-wave pulse whose center frequency is "f", at a predetermined interval into the human body by a transducer; then it receives a reflected signal, i.e., an echo signal, from a moving reflective object, such as a blood corpuscle; and detects the amount of phase shift of the echo signal, i.e., Doppler shift. An output signal of the phase shift amount, i.e. Doppler signal, indicates blood flow velocity.
In the Doppler blood flow velocity detection apparatus, the relation between a shift frequency fd of a Doppler signal and blood flow velocity V is given by: EQU fd=(2V/c).multidot.f cos .theta. (1)
where "c" is a sound velocity in the human body; .theta. is an angle made by the ultrasonic transmitting direction with the direction of blood flow, wherein the shift frequency fd is subjected to a limitation given by: EQU .vertline.fd.vertline..ltoreq.fp/2 (2)
where fp is a repetition frequency of ultrasonic-wave pulse (also referred to as a rate frequency). The Doppler shift frequency fd should not exceed a half of the frequency fp because of the sampling theory. If blood velocity V exceeds a velocity corresponding to ultrasonic-wave pulse repetition frequency fp, the ultrasonic Doppler blood flow velocity detection apparatus outputs incorrect velocity and direction. Particularly, if a deep portion is measured, the period of time from transmission of an ultrasonic-wave pulse to reception of the reflected ultrasonic waves by a sensor of the apparatus becomes long. Then, the frequency fp of the ultrasonic-wave pulse should be set to a low value. Therefore, it is difficult to detect a high velocity of blood flow.
An ultrasonic Doppler blood velocity detecting method is disclosed in the technical report of the Institute of Electronics, Information and Communication Engineers, Vol. 87, No. 294, U.S. 87-51, 1987, which is provided to moderate the limitation of measurable blood flow velocity.
FIG. 6 shows a waveform of transmitted ultrasonic-wave pulses according to the above-mentioned prior art. In FIG. 6, ultrasonic-wave pulses are outputted repeatedly at intervals T and T+Ts alternately. The echo signal has phase shift .DELTA..theta. when an ultrasonic-wave pulse is transmitted which has the interval T to the subsequent pulse and phase shift .DELTA..theta.' when an ultrasonic-wave pulse is transmitted which has the interval Ts to the subsequent pulse. A velocity of blood flow is given by a phase shift .DELTA..DELTA..theta. which is obtained by .DELTA..theta.'-.DELTA..theta.. The measurable range is given by EQU .vertline.f.sub.d .vertline..ltoreq.(1/2).multidot.(1/Ts):
Therefore, decrease in Ts extends measurable range of blood flow velocity in consideration of Equation (1).
However, there is a drawback that the ultrasonic Doppler blood velocity detection apparatus according to the above-mentioned method is complicated because ultrasonic-wave pulses should be transmitted at two different intervals and a phase shift calculation is required to obtain the phase shift .DELTA..DELTA..theta.. The detection of the phase shift .DELTA..DELTA..theta. should be measured repeatedly to obtain a means value because it is not accurate. Therefore, there is also a drawback that it is impossible to obtain an instantaneous value of the blood flow velocity.